Stone columns are simply vertical columns of compacted crushed stone, gravel or sand which extend through a deposit of soft material or soil to be strengthened. In general, a number of these densely compacted granular material columns are produced beneath the site for the intended construction project. These columns serve to stabilize the soil, resulting in considerable vertical load capacity and improved shear resistance in the soil mass. The stone columns improve drainage in fine grained soil deposits and increase load bearing capacity to a point where considerably larger bearing stresses may be sustained without causing detrimental or excessive settlement or bearing capacity failure in the ground.
The following applications have used stone columns for soil stabilization to limit loose dirt settlement under reinforced earth walls, tank farms, dam and highway embankments, bridge abutments, and buildings. Another application is the stabilization and prevention of landslides. Stone columns also function as efficient gravel drains in providing a path for relief of excess pore water pressures, thus preventing liquefaction during an earthquake.
Although there are a number of well-known methods for the formation of stone columns in the ground. A common method is the use of a special vibrator, sometimes known as a Vibroflot, which expels water from its body as it sinks into the ground, thus forming a hole which is held open by water pressure and then filled with stone and the stone is compacted into the ground in stages using the vibrator. An example of this method is described in U.S. Pat. No. 4,397,588 for method of constructing a compacted granular or stone column in soil masses and apparatus. This method is more commonly know as the Wet Top Feed Method.
Utilization of this method produces very large quantities of soil laden water by-product which must be disposed of. Disposal of this by-product is difficult and expensive under the best of conditions, and virtually prohibitive at environmentally sensitive locations. Consequently, most column installations with vibrators now make use of ancillary bottom-feed equipment which provides a feed pipe to the tip of the vibrator. Stone is fed through this pipe to the tip of the vibrator using compressed air, thus eliminating the need for water. Although production by this method is somewhat slower, and thus slightly more expensive, savings in the disposal of the by-product usually more than offsets the additional cost. This method is more commonly know as the Dry Bottom Feed Method.
Dry Bottom Feed stone columns are a soil improvement technique for such applications where the Wet Top Feed Method is not feasible due to problems related to the by product of flushing water that is produced with the Wet Top Feed Method.
Both the Wet Top Feed technique and the Dry Bottom Feed technique are both suitable for installing stone columns which can be used to reduce the effects of differential soil settlements and accelerates the time which cohesive soils require to consolidate.
Since soil with a fines content of less than 12% will compact easier with the vibrations associated with column installation, the stone column method (dry or wet) is ideal for the prevention of soil liquefaction in the event of an earthquake by compacting granular layers of soil. Stone columns also reinforce and drain cohesive soils at the same time.
The dry bottom feed method occurs when a stone column is installed in a way that the gravel is transported without the use of water in the special duct alongside the vibratory mechanism.
The Dry Top Feed Method is the same as the Dry Bottom Feed Method; however, in the dry top feed method the gravel is dumped into the open hole while the vibratory mechanism is fully retracted. This method works only if the hole does not collapse, i.e. instable soil above ground water table.
The wet top feed method installation occurs when the gravel is added from the top of the hole using flushing water to keep open an annular space around the vibratory mechanism.
The wet bottom feed method installation occurs when a stone column is installed in a way that the gravel is transported to the tip of the vibratory mechanism with the aid of water in the special duct alongside the vibratory mechanism.
Re-penetration occurs during the installation of stone columns, when the vibratory mechanism is lowered back into the gravel to enlarge the column diameter to the desired value.
The vibratory mechanism is typically utilized to construct a sand or stone column, wherein the probe itself generally consists of a 12 to 16 inch diameter hollow cylindrical body. In general, the vibrator is powered by a motor (electric or hydraulic) rated at a minimum of 130 kW and a minimum centrifugal force of 210 kN gyrating about a longitudinal axis to create lateral vibrations in the probe, i.e., vibrations in the horizontal plane. The minimum double amplitude (peak to peak measurement) of the probe tip is not less than 18 mm in a horizontal direction when the probe is in a free suspended position. The probe is constructed with follower tubes, electric cables, and/or hydraulic hoses. Water hoses can also be connected to the uppermost extension tube. The complete assembly of the vibrator probe (the vibratory mechanism) is usually supported from a commercial crane.
In an effort to construct a stone column utilizing this type of probe, wherein the probe is penetrated into the soil to a predetermined depth under its own weight and with vibration and assistance of a jetting fluid, the jetting fluid, which is under pressure, may be compressed air or water. At the required depth, the probe is slowly retrieved in small increments to allow backfill material to be placed under the space left by the withdrawn tip of the probe. Granular backfill material is transported to the probe tip through a transfer pipe that runs parallel to the probe down to the tip of the nosecone. The transfer pipe is fed by a hopper, typically mounted at the top of the probe assembly. As the probe is withdrawn, the granular material is fed into the void by the probe tip. As the probe is partially lowered again with the vibrating force, the freshly deposited granular material in the surrounding soil becomes compacted. By repetition of these steps, the stone column is gradually constructed.
It is universally recognized that a multi chute stone feeding system would increase production rates, reduce cost, and improve quality. But, unfortunately the industry has not been able to achieve any success in operating any twin feeding system because of constant blockages and break downs of such apparatus. As a result, the industry has move away from that system and has adopted the single pipe feed system. As such, there is a need in the art for a multi feeding system that has two or more feeding chutes, which can increase production rates, reduce cost, and improve quality of the stone columns, and which does not have blockage problems.
The twin chute feed system has been attempted in the past but was not successful largely because if one chute became slightly blocked, air flow was reduced by the blockage at a point in time when it was necessary to have it increased in order to remove the blockage. This had a cascading effect where the flow through the blocked chute became smaller and smaller and the flow through the clear chute became correspondingly larger. Eventually one chute blocked completely. Thus, there is a need in the art for a new method & apparatus that can overcome the above problems by constantly monitoring and adjusting the flow of air and pressure of the air in: (1) stone chamber, (2) stone feed pipe (3) transition splitter chute, and (4) multiple feed chutes attached to vibratory mechanism through either a hand operated or fully automated control system.
In previous systems, if debris became stuck to the stone valve the usual way to clean the valve was to open and close it several times in the hopes that it would dislodge the debris and clear itself. As such, there is a need in the art for a new device that can overcome the above problems.
Stone capacity of the hopper is one of the factors which directly limits the stone delivery system. The size of the hopper is limited by the need to keep the equipment within legal load limits for road transportation.
The industry has chosen to limit the width of the stone skip to the same width or smaller than the stone hopper which in turn limits the width of the loader which is used to fill the skip to the same or smaller width as the skip. The industry average capacity is about 2 cycles per charge of stone. Thus, there is a need in the art for a new method & apparatus that can overcome the above problems and limitations to increase the stone storage capacity of the skip.